Project Summary Viruses elicit multifaceted responses from the cells that they infect. Among the antiviral defenses of the host cell is activation of a DNA damage response. In the case of adenovirus (Ad) infection, the double-stranded, linear DNA genomes represent substrates for cellular DNA repair pathways that result in Ad genomes being joined into concatemers during infection with viruses deleted of the E4 open reading frames. Concatemerization requires host DNA repair proteins, including the Mre11/Rad50/Nbs1 complex (MRN). Infection with E4-deleted Ad is accompanied by activation of the cellular DNA damage signaling cascades. The DNA damage response is inactivated during wild-type Ad infection by the products of the early region E4. The E4orf3 protein forms intranuclear track structures, which sequester the MRN proteins and prevent ATR damage signaling. The E4orf6 protein forms a complex with E1b55K that recruits cellular proteins to form an E3 ligase that targets cellular repair factors for degradation, including MRN and ligase IV proteins. This proposal probes mechanistic aspects of the cellular response to adenovirus infection and its impact on the viral lifecycle. We show that MRN inhibits virus DNA replication, identify a role for the CtIP protein, and suggest a model that involves removal of the terminal protein from the virus genome. In the first Aim we will address the impact of the cellular repair proteins on virus infection and test the model for inhibition. The subsequent two Aims will address ways in which E4 proteins counteract these host antiviral functions. In Aim 2 we will study the mechanism used by E1b55K/E4orf6 to target cellular factors for proteasomal degradation, and the impact on virus replication. Aim 3 will focus on Ad E4orf3, and will investigate the correlation between disruption of PML bodies, mis-localization of the MRN complex, prevention of concatemer formation, inhibition of damage signaling, and production of late proteins by E4orf3. These studies will elucidate the biological relevance of the interactions between cellular repair factors and viral proteins, and will have broader implications for our understanding of cellular DNA damage responses.